The present invention relates generally to optically phased laser telescope systems, and more specifically to an electronic circuit which drives an optical path difference adjustor to bring separate laser beams into phase with each other by adjusting the length of their optical paths.
The optical phasing of separate transmitted beams of laser transmitters with monochromatic light sources is achieved by matching the optical paths. An excellent example of a successful system which uses an array of multiple optical laser telescopes to achieve the performance of a single laser transmitter of equivalent size, is disclosed in U.S. Pat. No. 4,639,586, issued Jan. 27, 1987 to Janet S. Fender et al, entitled "An Optically Phased Laser Transmitter."
The Fender apparatus performs phase matching between pairs of laser beams using an array containing at least two optical telescopes which become useable as a laser transmitter when combined with an optical phase matching system consisting of: a collecting telescope, a detector array, two fold mirrors, analog-to-digital (A/D) converter, microprocessor, and two sets of correcting mirrors.
The two optical telescopes are adjacent to each other, and transmit two separate outgoing laser beams which require phase matching. The original source of the two outgoing beams may be either: a single laser beam, which has been divided (monochromatic); or two separately transmitted polychromatic laser beams.
The collecting telescope sits in front of the two optic telescopes and bridges the gap between them. In this way, the collecting telescope is able to intercept samples of outgoing laser beams from the edges of both telescopes and focus them, through the two fold mirrors to the detector array.
The detector array may be either a line scan or an area charge coupled device (CCD), which reads out the fringe pattern by generating an interference pattern.
The A/D converter, microprocessor, and correcting mirrors are used to match the phase of the outgoing beams by adjusting the optical path lengths of the beams. The above-cited Carreras application discloses a system which performs the functions attributed to the analog-to-digital converter and microprocessor in the Fender application. The Carreras system receives the interference pattern between samples of pairs of transmitted laser beams from a CCD camera, then performs a calculation of the difference in optical path lengths between the two beams which allows the laser transmitter to match the phase of the outgoing beams.
The above-cited application of Carreras et al discloses an electrical tracking system, which is used to achieve simultaneous low-bandwidth tracking of the tilt-induced errors on the signal beams from three independent telescopes. Each signal beam is passed through the same rotating chopper wheel which includes strategically placed sampling apertures or holes to allow the sequential sampling of the return beams. The sampled beams are then focused onto a single photo-detector cell wherein the beams are multiplexed, thus allowing a single detector to sample all the return beams while providing a single common reference point. The sampled beam is combined with the direct return beam signal from each telescope to provide an error tilt compensation signal.
While phased array radar systems have been around for years, optically phased laser transmitters are a comparatively newly emerging technology. Among the needs of current systems are the need for improved methods of correcting optical path length adjustments. The task of correcting optical path lengths is alleviated to some extent, by the systems of the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 3,599,112 issued to Holtz;
U.S. Pat. No. 4,239,392 issued to Pohle;
U.S. Pat. No. 4,326,800 issued to Fitts;
U.S. Pat. No. 4,384,198 issued to Williamson; and
U.S. Pat. No. 4,413,909 issued to Pohle.
Pohle U.S. Pat. No. 4,413,909 discloses a system for measuring and correcting for tilt and aberrations in a laser beam. In the patent reference subaperture wavefront is compared with other subaperture wavefronts. The subapertures are focused on a diffraction grating to form images with interference patterns. A measurement is made of the phase difference between intensity fluctuations of the interference patterns. A similar wavefront sampling system for a laser beam projector or telescope is disclosed in Pohle U.S. Pat. No. 4,239,392.
Fitts is concerned with an automatic alignment system for high energy lasers. A servo-control system includes a wavefront sensor which analyzes the wavefront profile of a low energy replica of the high energy beam and generates control signals which actuate a deformable mirror to correct spurious wavefront aberrations. Williamson shows a time shared aperture device with wavefront analyzers and Holtz is included for its disclosure of a laser generator with a telescopic optical system.
While the above cited references are instructive, a need remains for a mechanism to remove detected errors in optically phased laser transmitters. The present invention is intended to satisfy that need.